The use of floating roof tanks for the storage of crude oil, gasoline and other volatile liquids is well known. To prevent the escape of vapors from the liquid stored in the tank to the atmosphere some type of sliding seal arrangement is provided to seal the annular gap between the periphery of the floating roof and the surrounding cylindrical shell of the tank. Because the walls of the tank are not perfectly smooth and because the cross-sectional shape of the tank departs from a true circle with changes in temperature, wind loads, and with shifting of the sub-surface material under the substantial weight of the stored liquid, such known sliding seals permit varying amounts of vapors to escape into the atmosphere at various roof elevations and at varying ambient conditions. Restrictions imposed by state and federal government agencies to restrict emissions into the atmosphere have imposed gap tolerances for the sliding seals which are difficult to meet and maintain. Even allowable emissions from sliding seals degredate ambient air quality to an extent in many areas such that closed roof tankage with expensive vapor recovery systems, or emission trade-offs with other industries, are necessary.